1. Field of the Invention
This invention relates to a method of preparing a metal oxide crystal and, more specifically, to a method for the growth of a metal oxide crystal.
2. Description of Prior Art
Known methods for the preparation of superconducting metal oxides of a Y-series 123 structure include a flux method, a melt solidification method, a flowing zone (FZ) method, a unidirectional solidification method and a crystal pulling method.
In the flux method, the growth of the metal oxide crystal is performed by lowering the temperature of a relatively homogeneous solution. An improvement of the flux method is suggested in Physica C, 190, 64 (1991), in which peritectic reaction and diffusion in a liquid phase are utilized. The flux method has a problem because it is impossible to continuously grow the desired crystal to a large crystal.
The melt solidification method is a method including the steps of heating a desired metal oxide or a precursor thereof at above the peritectic temperature thereof to form a mixed phase in which a solid phase formed by decomposition and a liquid phase coexist, and then cooling the mixed phase to form the desired crystal of the metal oxide (Jpn. J. Appl. Phys. 28, 1125 (1989); Advances in Superconductivity II, Springer-Verlead, Tokyo, p. 285). While a relatively large crystal can be obtained by this method, it is impossible to continuously prepare the desired crystal without inclusions.
In the unidirectional solidification method, of a raw material in the form of a bar is heated above its peritectic temperature while displacing the heating zone from one end thereof to the other end. While this method permits the continuous preparation of the crystal, it is difficult to form a single phase because the composition of the heating portion varies with the displacement of the high temperature liquefied region. In particular, the desired crystal is apt to be contaminated by a different crystal phase produced by the peritectic reaction.
The FZ method includes heating a raw material in the form of a bar at a higher temperature than that adopted in the above unidirectional solidification method while displacing the heating zone from one end thereof to the other end. This method has a problem because it is difficult to prepare a large crystal. Further, this method is not applicable to the preparation of a Y-series 123 metal oxide crystal, because at such a high temperature, the fused mass begins flowing.
The crystal pulling method includes the steps of providing a solution of the desired crystals in a suitable solvent, immersing a bar of a seed crystal, having a lower temperature than that of the solution, in the solution to form deposits of the desired crystal on the immersed region of the bar, and gradually pulling up the bar to permit the crystal to grow. With this method, however, it is difficult to form a Y-series 123 metal oxide crystal. Further, this method, which uses the primary crystallization temperature, has a problem because the solution has a high viscosity.